Build: add CSS transitions to Modernizr build
Session: updated English demo, added French demo
Session: added modal dialog for timeout warning

1. Modal built using Magnific Popup and exposes logic via
   `modal.wb-session-timeout` event.
2. Countdown of reaction time displayed in modal.  Once zero
   is reached, session expired message and "Sign in" button displayed.
Merge pull request #3213 from patheard/session-timeout

Session timeout modal dialog